1. Field of the Invention
This application relates generally to the treatment of cancer. Specifically, the invention relates to novel compounds useful for chemotherapy, methods of synthesis of these compounds, and methods of treatment employing these compounds. The novel compounds described are analogs of the doxorubicin which is known to have anti-tumor effects.
2. Description of the Related Art
Resistance of tumor cells to the killing effects of chemotherapy is one of the central problems in the management of cancer. It is now apparent that at diagnosis many human tumors already contain cancer cells that are resistant to standard chemotherapeutic agents. Spontaneous mutation toward drug resistance is estimated to occur in one of every 10.sup.6 to 10.sup.7 cancer cells; this mutation rate appears to be independent of any selective pressure from drug therapy, although radiation therapy and chemotherapy may give rise to additional mutations and contribute to tumor progression within cancer cell populations (Goldie et al., Cancer Treat. Rep., 63:1727, 1979; Goldie et al., Cancer Res., 44:3643, 1984; and Nowell, Cancer Res., 46:2203, 1986). The cancer cell burden at diagnosis is therefore of paramount importance because even tumors as small as 1 cm (10.sup.9 cells) could contain as many as 100 to 1,000 drug-resistant cells prior to the start of therapy.
Selective killing of only the tumor cells sensitive to the drugs leads to an overgrowth of tumor cells that are resistant to the chemotherapy. Mechanisms of drug resistance include decreased drug accumulation (particularly in multidrug resistance), accelerated catabolism of the drug and other alterations of drug metabolism, and an increase in the ability of the cell to repair drug-induced damage (Curt et al., Cancer Treat. Rep., 68:87, 1984; and Kolate, Science, 231:220, 1986). The cells that overgrow the tumor population not only are resistant to the agents used but also tend to be resistant to other drugs, many of which have dissimilar mechanisms of action. This phenomenon, called pleiotropic drug resistance or multidrug resistance (MDR), may account for much of the drug resistance that occurs in previously treated cancer patients.
Gene amplification (i.e., the production of extra copies of genes within a cell) is one of the mechanisms that can lead to drug resistance. Gene amplification is involved in the phenomenon of multidrug resistance. Multidrug resistance appears to be linked to over-expression of a cell membrane glycoprotein, termed P-glycoprotein, on the surface of cancer cells (Bell et al., J. Clin. Oncol., 3:311, 1985; and Bertino, J. Clin. Oncol., 3:293, 1985). The action of this glycoprotein is unknown, but its over-expression is associated with decreased accumulation of multiple chemotherapeutic drugs within the resistant cells. A multidrug-resistance gene that encodes the P-glycoprotein has been isolated and sequenced, and when it is transferred, this gene confers drug resistance on previously drug-sensitive cells (Gros et al., Nature, 323:728, 1986). The multidrug-resistance gene termed mdrl is expressed in several normal tissues, and its expression is increased in some human tumors (Fojo et al., P.N.A.S., 84:265, 1987). Various human tumors are now being analyzed to determine whether they express this gene. Because many heavily treated patients who are in relapse harbor tumors that do not show over-expression of the mdrl gene, it appears that other mechanisms are probably also involved in causing resistance to chemotherapy.
The commonly used chemotherapeutic agents are classified by their mode of action, origin, or structure, although some drugs do not fit clearly into any single group. The categories include alkylating agents, antimetabolites, antibiotics, alkaloids, and miscellaneous agents (including hormones); agents in the different categories have different sites of action.
Antibiotics are biologic products of bacteria or fungi. They do not share a single mechanism of action. The anthracyclines daunorubicin and doxorubicin (DOX) are some of the more commonly used chemotherapeutic antibiotics. The anthracyclines achieve their cytotoxic effect by several mechanisms, including intercalation between DNA strands, thereby interfering with DNA and RNA synthesis; production of free radicals that react with and damage intracellular proteins and nucleic acids; chelation of divalent cations; and reaction with cell membranes. The wide range of potential sites of action may account for the broad efficacy as well as the toxicity of the anthracyclines (Young et al., N. Engl. J. Med., 312:692, 1985).
The anthracycline antibiotics are produced by the fungus Streptomyces peucetius var. caesius. Although they differ only slightly in chemical structure, daunorubicin has been used primarily in the acute leukemias, whereas doxorubicin displays broader activity against human neoplasms, including a variety of solid tumors. The clinical value of both agents is limited by an unusual cardiomyopathy, the occurrence of which is related to the total dose of the drug; it is often irreversible. In a search for agents with high antitumor activity but reduced cardiac toxicity, anthracycline derivatives and related compounds have been prepared. Several of these have shown promise in the early stages of clinical study, including epirubicin and the synthetic compound mitoxantrone, which is an amino anthracenedione.
The anthracycline antibiotics have tetracycline ring structures with an unusual sugar, daunosamine, attached by glycosidic linkage. Cytotoxic agents of this class all have quinone and hydroquinone moieties on adjacent rings that permit them to function as electron-accepting and donating agents. Although there are marked differences in the clinical use of daunorubicin and doxorubicin, their chemical structures differ only by a single hydroxyl group on C14. The chemical structures of daunorubicin and doxorubicin are as follows: ##STR1##
Unfortunately, concomitant with its antitumor activity, DOX can produce adverse systemic effects, including acute myelosuppression, cumulative cardiotoxicity, and gastrointestinal toxicity (Israel et al., Cancer Treat. Rec., 14:163, 1987). At the cellular level, in both cultured mammalian cells and primary tumor cells, DOX can select for multiple mechanisms of drug resistance that decrease its chemotherapeutic efficacy. These mechanisms include P-gp-mediated MDR, characterized by the energy-dependent transport of drugs from the cell (Bradley et al., Biochem. Biophys. Acta., 948:87, 1988), and resistance conferred by decreased topoisomerase II activity, resulting in the decreased anthracycline-induced DNA strand scission (Danks et al., Cancer Res., 47:1297, 1987; Pommier et al., Cancer Res.), 46:3075, 1986; Moscow et al., J. Natl. Cancer Inst., 80:14, 1988.
Among the potential avenues of circumvention of systemic toxicity and cellular drug resistance of the natural anthracyclines is the development of semisynthetic anthracycline analogues which demonstrate greater tumor-specific toxicity and less susceptibility to various forms of resistance. One such analogue, AD 198, exhibits a variety of mechanistic differences compared with anthracycline, including weaker binding to purified DNA, preferential inhibition of RNA versus DNA synthesis, irreversible G.sub.2 /M blockade, pronounced membrane lytic activity, and a lack of inhibition of purified mammalian topoisomerase II despite significant levels of protein-associated DNA strand breaks in alkaline elution assays (Israel et al., 1987; Traganos et al., Cancer Res., 45:6273, 1985; Bodley et al., Cancer Res., 49:5969; and Israel et al., Cancer Chemother. Pharmacol., 25:177, 1989). When compared with anthracycline, AD 198 demonstrates enhanced cytotoxicity against cultured murine and human tumor cells and the ability to circumvent MDR in P388 and L1210 leukemic cells and B16-BL6 melanoma cells, and both MDR and resistance due to altered topoisomerase II activity in variant CCRF-CEM leukemic cells (Ganapath, et al., Br. J. Cancer, 60:819, 1989; Sweatman et al., J. Cell. Pharmacol., 1:95-102). Unfortunately, despite the high degree of toxicity seen in vitro, AD 198 exhibits limited efficacy against transplanted MDR L1210 cells in vivo. This observation suggests that resistance to AD 198 may be conferred either systemically through enhanced drug metabolism or pharmacologic sanctuary of the neoplasia or through cellular resistance (Ganapathi et al.).
AD 198 is an anthracycline analogue designed to circumvent MDR and thereby enhance chemotherapeutic efficacy against drug-resistant neoplastic cells. Circumvention of MDR by AD 198 appears to be due, at least in part, to the inability of P-gp to transport AD 198 from the cell (Sweatman et al.). Cellular resistance to AD 198 can emerge rapidly within murine macrophage-like J774.2 cells and with characteristics that were similar to those of MDR. However, AD 198.sup.R cells differ from MDR cells in the persistence of high levels of intracellular AD 198 similar to that of drug-sensitive cells. Resistance to AD 198 could limit the successful utilization of this and other highly hydrophobic anthracycline analogues in the treatment of MDR tumor cells. In addition, it has been shown that MDR J774.2 selected with vinblastine were cross-resistant to AD 198. This finding was in contrast to previous studies showing the ability of AD 198 to circumvent MDR in lymphocytic and melanoma cell lines selected with either anthracycline or vinblastine (Ganapathi et al.; Sweatman et al.), suggesting that cell type may have a significant effect upon the MDR phenotype which emerges in response to challenge with a particular drug.
A further example of cell type effecting the drug resistance phenotype with regard to a particular drug is seen in the various particular AD 198 resistant cells. Two types of AD 198-resistant cells are the AD 198.sup.R cells, which are created by selecting AD 198 resistant cells from the normally AD 198 susceptible J774.2 cells, and the A300 cells, which are created by selecting cells with even greater resistance to AD 198 than the normally resistant A100 cells (Lothstein et al. Cancer R., 52:3409 (1992). Although both AD 198.sup.R cells and A300 cells are resistant to AD 198, the basis of the resistance appears to be different between the cell types (Lothstein).
The development of drug resistance is one of the major obstacles in the management of cancer. There are various types of drug resistance, for example, classic MDR as opposed to AD 198 resistance. Furthermore, different cell lines can establish resistance to the same drug in different ways, as seen in the case of the differences in AD 198 resistance in AD 198.sup.R cells as opposed to A300 cells. One of the traditional ways to attempt to circumvent this problem of drug resistance has been combination chemotherapy.
Combination drug therapy is the basis for most chemotherapy employed to treat breast, lung, and ovarian cancers as well as Hodgkin's disease, non-Hodgkin's lymphomas, acute leukemias, and carcinoma of the testes.
Combination chemotherapy uses the differing mechanisms of action and cytotoxic potentials of multiple drugs. Although all chemotherapeutic drugs are most effective on cells that are active in DNA synthesis, many agents--particularly the alkylating agents--can kill cells that are not cycling. Such agents are termed non-cell proliferation-dependent agents can shrink tumor mass by reducing cell numbers; the surviving cells will then move into the cycling compartment, where they are more susceptible to cell proliferation-dependent drugs. The combined use of agents less dependent on the cell cycle followed by those dependent on cell proliferation is effective in enhancing tumor cell death. Each cycle of treatment kills a fixed fraction of cells, so repetitive cycles are required for cure. For example, a drug combination that kills 99.9 percent of cancer cells per treatment cycle would have to be repeated at least six times to eliminate an average tumor burden (if tumor cells did not regrow between cycles).
Although combination chemotherapy has been successful in many cases, the need still exists for new anti-cancer drugs. These new drugs could be such that they are useful in conjunction with standard combination chemotherapy requires. Or, these new drugs could attack drug resistant tumors by having the ability to kill cells of multiple resistance phenotypes. For example, a drug that has the ability to kill cells with both MDR and AD 198 resistance could eliminate two populations of resistant cells from a tumor.
A drug that exhibits the ability to overcome multiple drug resistances could be employed as a chemotherapeutic agent either alone or in combination with other drugs. The potential advantages of using such a drug in combination with chemotherapy would be the need to employ fewer toxic compounds in the combination, cost savings, and a synergistic effect leading to a treatment regime involving fewer treatments.